The present invention relates to a variable displacement compressor used in vehicle air conditioners. Specifically, the present invention pertains to a device and a method for controlling the displacement of a variable displacement compressor.
FIG. 12 shows a prior art variable displacement compressor. The compressor includes a housing 101. A crank chamber 102 is defined in the housing 101. A drive shaft 103 is supported in the housing 101. A lip seal 104 is located between the housing 101 and the drive shaft 103 to prevent gas leakage along the surface of the drive shaft 103.
The drive shaft 103 is connected to a vehicle engine Eg, which serves as an external power source, through an electromagnetic friction clutch 105. The friction clutch 105 includes a pulley 106, an armature 107 and an electromagnetic coil 108. The pulley 106 is coupled to the engine Eg, and the armature 107 is coupled to the drive shaft 103. When the clutch 105 engages, that is, when the coil 108 is excited, the armature 107 is attracted to and is pressed against the pulley 106. As a result, the clutch 105 transmits the driving force of the engine Eg to the drive shaft 103.
When the clutch 105 disengages, that is, when the coil 108 is de-excited, the armature 107 is separated from the pulley 106. In this state, the driving force of the engine Eg is not transmitted to the drive shaft 103.
A rotor 109 is secured to the drive shaft 103 in the crank chamber 102. A thrust bearing 122 is located between the rotor 109 and the inner wall of the housing 101. A swash plate 110 is coupled to the rotor 109 by a hinge mechanism 111. The hinge mechanism 111 permits the swash plate 110 to rotate integrally with the drive shaft 103 and to incline with respect to the axis L of the drive shaft 103. A limit ring 112 is fitted about the drive shaft 103. When the swash plate 110 abuts against the limit ring 112 as illustrated by broken lines in FIG. 12, the swash plate 110 is at the minimum inclination position.
Cylinder bores 113, suction chamber 114 and a discharge chamber 115 are defined in the housing 101. A piston 116 is reciprocally housed in each cylinder bore 113. The pistons 116 are coupled to the swash plate 110. The housing 101 includes a valve plate 117. The valve plate 117 separates the cylinder bores 113 from the suction chamber 114 and the discharge chamber 115.
Rotation of the drive shaft 103 is converted into reciprocation of each piston 116 by the rotor 109, the hinge mechanism 111 and the swash plate 110. Reciprocation of each piston 116 draws refrigerant gas from the suction chamber 114 to the corresponding cylinder bore 113 via a suction port 117a and a suction valve flap 117b, which are formed in the valve plate 117. Refrigerant gas in each cylinder bore 113 is compressed to reach a predetermined pressure and is discharged to the discharge chamber 115 via a discharge port 117c and a discharge valve flap 117d, which are formed in the valve plate 117.
A spring 118 urges the drive shaft 103 forward (to the left as viewed in FIG. 12) along the axis L through a thrust bearing 123. The spring 118 prevents axial chattering of the drive shaft 103.
The crank chamber 102 is connected to the suction chamber 114 by a bleeding passage 119. The discharge chamber 115 is connected to the crank chamber 102 by a supply passage 120. The opening of the supply passage 120 is regulated by an electromagnetic displacement control valve 121.
The control valve 121 adjusts the opening of the supply passage 120 to regulate the amount of pressurized refrigerant gas drawn into the crank chamber 102 from the discharge chamber 115. The pressure in the crank chamber 102 is changed, accordingly. Changes in the crank chamber pressure alter the gas pressure moment acting on the pistons 116 through the swash plate 110, which changes the inclination of the swash plate 110. Accordingly, the stroke of each piston 116 is changed and the compressor displacement is varied. The gas pressure moment depends on the crank chamber pressure and the pressure in the cylinder bore 113, which act on the pistons 116.
When the clutch 105 disengages or when the engine Eg is stopped, the control valve 121 fully opens the supply passage 120, which increases the pressure in the crank chamber 102. Accordingly, the gas pressure moment decreases the inclination of the swash plate 110. The compressor stops operating with the swash plate 110 at the minimum inclination position. When the compressor is started again, the displacement of the compressor is minimum, which requires minimum torque. The shock caused by starting the compressor is thus reduced.
When there is a relatively great cooling demand on a refrigeration circuit that includes the compressor, for example, when the temperature in a passenger compartment of a vehicle is much higher than a target temperature set in advance, the control valve 121 closes the supply passage 120 and maximizes the compressor displacement.
When the clutch 105 disengages or when the engine Eg is stopped, the compressor is stopped. If the compressor is stopped when operating at the maximum displacement, the control valve 121 quickly and fully opens the supply passage 120, which was fully closed. Accordingly, highly pressurized refrigerant gas in the discharge chamber 115 is quickly supplied to the crank chamber 102. Refrigerant gas in the crank chamber 102 constantly flows to the suction chamber 114 through the bleeding passage 119. However, since the amount of refrigerant gas that flows to the suction chamber 114 through the bleeding passage 119 is limited, the pressure in the crank chamber 102 is quickly and excessively increased by as the supply passage 120 is quickly and fully opened. Also, when the compressor is stopped, the pressure in each cylinder bore 113 approaches the pressure in the suction chamber 114, which is relatively low. As a result, the gas pressure moment decreasing the swash plate inclination becomes excessive.
Accordingly, the swash plate 110 is moved from the maximum inclination position to the minimum inclination position and strongly presses the drive shaft 103 rearward (to the right as viewed in FIG. 12) through the limit ring 112. The swash plate 110 also strongly pulls the drive shaft 103 rearward through the hinge mechanism 111 and the rotor 109. The drive shaft 103 is thus moved rearward along its axis L against the force of the spring 118.
When the drive shaft 103 moves rearward, the axial position of the drive shaft 103 relative to the lip seal 104, which is retained in the housing 101, changes. Normally, a predetermined annular area of the drive shaft 103 contacts the lip seal 104. Foreign particles and sludge adhere to areas of the drive shaft 103 that are axially adjacent to the predetermined annular area. Therefore, if the axial position of the drive shaft 103 relative to the lip seal 104 changes, sludge enters between the lip seal 104 and the drive shaft 103. This lowers the effectiveness of the lip seal 104 and results in gas leakage from the crank chamber 102.
Particularly, when the drive shaft 103 moves rearward due to disengagement of the clutch 105, the armature 107, which is fixed to the drive shaft 103, moves toward the pulley 106. The clearance between the pulley 106 and the armature 107 is as small as 0.5 mm when the clutch 105 disengages. Rearward movement of the drive shaft 103 eliminates the clearance between the pulley 106 and the armature 107, which may cause the armature 107 to contact the rotating pulley 106. This produces noise and vibration. Also, even if the clutch 105 disengages, the driving force of the engine Eg may be transmitted to the drive shaft 103.
When the drive shaft 103 moves rearward, the average position of the pistons 116, which are coupled to the drive shaft 103 by the swash plate 110, is moved rearward. This causes the top dead center of each piston 116 to approach the valve plate 117. As a result, the pistons 116 may collide with the valve plate 117 when at their top dead center positions.
To prevent the drive shaft 103 from moving rearward, the force of the spring 118 may be increased. However, a greater spring force increases the load acting on the thrust bearings 122, 123 and increases the power loss of the compressor.
Accordingly, it is an objective of the present invention to provide displacement control device and method for variable displacement compressors that prevent a moment decreasing the inclination of the swash plate from being excessively increased.
To achieve the foregoing and other objectives and in accordance with the purpose of the present invention, a compressor for compressing gas supplied from an evaporator of an external refrigerant circuit and for discharging the compressed gas to the external refrigerant circuit is provided. The compressor includes a housing, a cylinder bore defined in the housing, a crank chamber defined in the housing and a suction chamber defined in the housing. The suction chamber is connected to the outlet of the evaporator. Gas is constantly released from the crank chamber to the suction chamber. The compressor further includes a piston, a drive shaft supported by the housing, a drive plate, a control valve and a check valve. The piston is accommodated in the cylinder bore and compresses gas drawn into the cylinder bore from the suction chamber and discharges the compressed gas from the cylinder bore. The drive plate is coupled to the piston to convert rotation of the drive shaft into reciprocation of the piston. The drive plate is supported by the drive shaft to incline relative to the drive shaft and is moved between a maximum inclination position and a minimum inclination position in response to a tilt moment acting on the drive plate. The tilt moment has components including a moment based on the pressure in the crank chamber and a moment based on the pressure in the cylinder bore. The inclination of the drive plate defines the stroke of the piston and the displacement of the compressor. The control valve controls the pressure in the crank chamber to change the inclination of the drive plate and is actuated based on an external command. The check valve is located between the suction chamber and the evaporator and is closed based on the pressure difference between the suction chamber and the outlet of the evaporator to prevent gas from flowing from the suction chamber to the evaporator.
The present invention may also be embodied in a displacement control valve for adjusting the pressure in a crank chamber of a compressor to change the displacement of the compressor. The compressor includes a suction pressure zone, the pressure of which is a suction pressure, a discharge pressure zone, the pressure of which is a discharge pressure, and a supply passage connecting the crank chamber to the discharge pressure zone. The control valve includes a valve body, a pressure sensing member and an electromagnetic actuator. The valve body adjusts the size of an opening in the supply passage. The pressure sensing member moves the valve body in response to the suction pressure to maintain the suction pressure at a predetermined target value. The electromagnetic actuator applies a force to the valve body. The force corresponds to the level of a current supplied to the actuator. The level of the current determines a target value of the suction pressure. The actuator increases the target value as the level of the current decreases and sets the target value to a maximum value when no current is supplied to the actuator.
The present invention may further be embodied in a method for controlling the displacement of a variable displacement compressor. The compressor includes a drive plate that is moved between a maximum inclination position and a minimum inclination position in accordance with the pressure in a crank chamber. The inclination of the drive plate defining the displacement of the compressor. The method includes: controlling the pressure in the crank chamber to change the inclination of the drive plate when the compressor is operating; increasing the pressure in the crank chamber to move the drive plate to the minimum inclination position when the compressor is stopped; and restricting an increase of the pressure in the crank chamber when a predetermined time has elapsed after the compressor is stopped.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.